Signpost
<p>
Connect all the squares together into a sequence, so that every
square's arrow points towards the square that follows it (though the
next square can be any distance away in that direction).

<p>
Left-drag from a square to the square that should follow it, or
right-drag from a square to the square that should precede it.

<p>
Left-drag a square off the grid to break all links to it. Right-drag
a square off the grid to break all links to it and everything else
in its connected chain.
